In this study, the three-dimensional (3D) nanoscale organization in the photoactive layers of poly(3-hexylthiophene) (P3HT) and a methanofullerene derivative (PCBM) is revealed by electron tomography. Morphologies suggested by previous experimental evidence were, for the first time, observed directly with a nanometer resolution and studied in detail. After annealing treatment, either at elevated temperature or during slow solvent evaporation, genuine 3D nanoscale networks are formed with high crystalline order and favorable concentration gradients of both P3HT and PCBM through the thickness of the photoactive layer. These favorable morphological changes account for a considerable increase of the power conversion efficiency in corresponding solar cell devices.

We present a combined numerical charge transport and morphology model to describe the current density-voltage ( j -V ) characteristics of three different, benchmark polymer:fullerene bulk heterojunction organic solar cells in which the device performance critically depends on the processing conditions or composition of the active layer. We fi nd that an accurate description of the j -V characteristics over a broad bias range can be obtained when the actual complex, three-dimensional (3D) phase separation is represented by a simplifi ed 2D or even 1D description. The morphological device model allows predicting the potential for increasing device performance by further optimizing the morphology. The optimal simplifi ed morphology consists of two, relatively thin alternating vertically oriented slabs, that allow for fast lateral separation of photocreated holes and electrons. This morphology can effectively be described as 1D.

We have analyzed the nanoscale organization of various polymer systems by utilizing high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM). All systems under investigation are purely carbon based; in some cases staining was used for comparison with conventional transmission electron microscopy (CTEM) imaging. For contrast creation we have applied density differences rather than differences in elemental composition of the materials. Because HAADF-STEM is an incoherent imaging technique, which provides images easy to interpret due to the lack of phase contrast, the high signal-to-noise ratio and the linearity of the signal intensity, imaging artifacts are substantially reduced and additional information on the nanoscale organization of polymer materials is obtained that is not accessible by CTEM. Exemplary, we present HAADF-STEM results form four different polymer systemssa rubber blend, a carbon black filled conductive nanocomposite, a functional blend as applied for the photoactive layer of a polymer solar cell, and semicrystalline polyethylenesand discuss critically contrast origin and the advantages of HAADF-STEM imaging for morphology characterization of polymer systems.

Nanostructured polymer-based solar cells (PSCs) have emerged as a promising low-cost alternative to conventional inorganic photovoltaic devices and are now a subject of intensive research both in academia and industry. For PSCs to become practical efficient devices, several issues should still be addressed, including further understanding of their operation and stability, which in turn are largely determined by the morphological organisation in the photoactive layer. The latter is typically a few hundred nanometres thick film and is a blend composed of two materials: the bulk heterojunction consisting of the electron donor and the electron acceptor. The main requirements for the morphology of efficient photoactive layers are nanoscale phase segregation for a high donor/acceptor interface area and hence efficient exciton dissociation, short and continuous percolation pathways of both components leading through the layer thickness to the corresponding electrodes for efficient charge transport and collection, and high crystallinity of both donor and acceptor materials for high charge mobility. In this paper, we review recent progress of our understanding on how the efficiency of a bulk heterojunction PSC largely depends on the local nanoscale volume organisation of the photoactive layer.